Changing Conditions on 55 Cancri e

byPaul GilsteronMay 5, 2015

Roughly twice the radius and eight times as massive as Earth, 55 Cancri e is a ‘super-Earth’ in the interesting five-planet system some 41 light years away in the constellation Cancer. No habitable conditions here, at least not for anything remotely like the kind of life we understand: 55 Cancri e orbits its G-class primary every 18 hours (55 Cancri is actually a binary, accompanied by a small red dwarf at a separation of 1000 AU). The closest super-Earth we’ve yet found, this is a tidally locked world that, helpfully for our purposes, transits its host.

What we find in a just announced study of the planet’s thermal emissions out of the University of Cambridge is an almost threefold change in temperature over a two year period. Although we’ve done it before with gas giant atmospheres, this is the first time any variability in atmosphere has been observed on a rocky planet outside our own Solar System. No other super-Earth has yet given us signs of possible surface activity, and Cambridge’s Nikku Madhusudhan, a co-author of the study, calls the changes in detected light ‘drastic.’ They imply a huge temperature swing, from 1000 degrees to 2700 degrees Celsius (∼1300 – 3000 K) on the star-side of this tidally locked world. Brice-Olivier Demory is lead author of the paper on these observations:

“We saw a 300 percent change in the signal coming from this planet, which is the first time we’ve seen such a huge level of variability in an exoplanet. While we can’t be entirely sure, we think a likely explanation for this variability is large-scale surface activity, possibly volcanism, on the surface is spewing out massive volumes of gas and dust, which sometimes blanket the thermal emission from the planet so it is not seen from Earth.”

Image: Artist’s impression of super-Earth 55 Cancri e, showing a hot partially-molten surface of the planet before and after possible volcanic activity on the day side. Credit: NASA/JPL-Caltech/R. Hurt.

The 55 Cancri e work was performed with data from the space-based Spitzer instrument. To understand the results, the authors look at the entire category of ultra-short period (USP) planet candidates found by Kepler, of which there are more than 100. Most of these have radii less than twice that of Earth, and some may be undergoing periods of intense erosion. The paper notes that the planet KIC12557548b shows changes in transit depth and shape that are consistent with what it calls a ‘cometary-like environment.’ The supposition here is that KIC12557548b is sub-Mercury in size but giving off an opaque cloud of dust, perhaps driven by surface volcanism.

From such scenarios the authors derive the idea that 55 Cancri e, one of the largest known USP planets, is likely subject to volcanism, with possible magma oceans on the day side. But in comparison to KIC12557548b, this world is large enough to contain its volcanic outgassing. From the paper:

…whereas extremely small planets (nearly mercury-size) subject to intense irradiation can undergo substantial mass loss through thermal winds, super-Earths are unlikely to undergo such mass-loss due to their significantly deeper potential wells… Thus, ejecta from volcanic eruptions on even the most irradiated super-Earths such as 55 Cnc e are unlikely to escape the planet and would instead display plume behaviour characteristic to the solar system. The extent and dynamics of the plumes if large enough can cause temporal variations in the planetary sizes and brightness temperatures and hence in the transit and occultation depths.

The larger picture is that we have begun to probe atmospheric conditions on worlds as small as two-Earth radii. Theories vary as to the composition of 55 Cancri e, with some observations suggesting a carbon-rich world while others point to a silicate-rich interior with a dense atmosphere. The variability found in this study calls the earlier models into question. But as we learn more about the material surrounding 55 Cancri e, we’ll be conducting what the authors call ‘a direct probe of the planet surface composition’ that may help us understand other USP planets.

The paper is Demory et al., “Variability in the super-Earth 55 Cnc e,” submitted to Monthly Notices of the Royal Astronomical Society (preprint). A University of Cambridge news release is available.

Comments on this entry are closed.

Harry R RayMay 5, 2015, 9:18

There has been a raging debate as to whether 55 Cancri e is a “carbon world or not (one of the authors of this paper FAVORING the “carbon world” interpretation). For volcanic ash to absorb that much heat and NOT re-radiate it means that the ash MUST be black as coal (OR BLACKER), giving MORE CREDENCE to the “carbon world” interpretation, although nowhere nearly PROVING it (obsidion would be a viable alternative, though not as likely,geologically speaking)! I can’t wait for 2018 to come around, when JWST will give us the answer!

Can we say now that 55 Cancri e does not have a THICK ( Hydrogen, water vapor, or a combination) atmosphere, whose pressure would not allow volcanic ash to rise high enough to insulate the planet as effectively as it appears to have?

Reminds me somewhat of the volcanic resurfacing that we believe happens on Venus every few tens of millions of years… just millions of times more frequent and on an almost-Neptune scale!

The deep potential well of this world was mentioned for its ability to keep hold of ejecta from the eruptions. Would be planet’s gravity also be subtantial enough for it to grab hold of material thrown out from its super-close parent star, such that the planet slowly gains mass over time? ( kind of the opposite of what happens to volcanic Io)

Well said Lionel. If you want to know more about ” stagnant lid” volcanism I would direct you too Edwin Kites excellent 2008 article “Geodynamics and rate of volcanism on Massive Super Earth Planets” . Available through arxiv. A little old but the salient points are still valid and it’s very readable . A great introduction to the subject. Lots of information without getting too technical. Gives reason for hope for the poor planets getting zapped by Giga years of M dwarf EUV and CMEs, but standing firm till the storm abates and then having the juice left to produce those crucial secondary atmospheres.
The take away message is the critical role water plays in driving tectonics through shearing in the outer mantle, lithosphere ,and the argument about how much of a planet’s internal heat is left over from creation as opposed to its radio nucleotide content decay ( U238, U235 , Thorium( mass no eludes me temporarily ) and Potassium 40 . Figures vary from 80:20 in favour of nucleotides to 20:80 in favour of internal heat ! Worth bearing in mind that bigger mass equals bigger planet equals smaller surface area to volume ratio by which to lose that heat. Rene Heller may have been right about that element of “superhabitability” . Certainly true for planets with low stellar irradiation or conversely torridly high irradiation .
A great paper covering planetary age , photosynthesis and habitability in arxiv today by the way. Well worth a read .

We may soon no longer need to rely just on the JWST thanks to two very recent related scientific breakthroughs. High dispersion and cross correlation spectroscopy. Neither require an exoplanetary transit .

In essence starlight reflects off a star and from our distant viewpoint we see the original light and its reflection as one light source. The starlight of course swamps the reflected light. The reflected light is almost the same as the direct starlight , but not quite (” modoki” as they say in Japanese) containing spectral lines of elements and molecules in the exoplanet atmosphere. How to sepetate them though ?!

We all know about the Doppler effect and its role in discovering orbiting exoplanets by pulling on the parent star both away and toward us during its orbit , an occurrence that can be spotted by sensitive spectroscopes and used to identify and partially characterise the exoplanet.

This process has now been taken a clever step further. The starlight reflected from the exoplanet is Doppler shifted itself compared to the original starlight , making it slightly different from the original starlight and enabling high resolution soectrographs like HARPS on the 3.6m La Silla telescope to separate it out of the overwhelming mass of direct starlight. The high dispersion of the light allows for greater numbers of spectral lines to be displaced just that little bit necessary to build an accurate spectroscopic map. Incredible.

The principle has been around since the 1950s but it’s only been in the last few years that three critical progenitors have come together. High resolution spectrographs ,extremely powerful supercomputers and their massive processing power to help seoerate the myriad of tight lines produced by the spectrograph and very high resolution, cooled and stabilised spectroscopes themselves . But it’s been done and even more importantly just on the small 3.6m La Silla telescope , not even one of the newer 8m class scopes.

The cross correlation function spectroscopy employs a “binary mask” that aids the process further. This method was covered in a great article on the site last week though it’s so sensational its implications take a while to sink in ! A measure of the recency of this innovation is that the excellent “Exoplanet handbook” published in 2014 doesn’t even anticipate it.

The plan now is to move the process to the larger multiple 8.2m ESO very large telescope array to increase its potency and better still from 2016/2017 combine the large telescope/s with the new “laser comb “very high resolution ESPRESSO 3rd generation spectrograph. This will then be used to perfect the technique and allow it to evolve as hardware and especially computers improve even further.

From 2024 the E-ELT ( and other ELTs with their own laser comb spectroscopes ) will come on line with the huge light gathering capacity of a 40m telescope in combination with the exquisitely sensitive 4th generation CODEX spectrograph and a new bespoke package called METIS , that will enable all of this to operate together not just in visible light but extending down to the mid infrared to give even more information.. If Moores law holds true ( and shows no sign of abating yet ) in ten years processing power will have increased by a further 8-16 times ,so in combination with CODEX and the E-ELT we will get incredibly detailed exoplanet atmospheric spectra, revealing even wind speed, clouds and weather . Probably a great deal more that no one had thought of such is the nature of big breakthroughs and especially in astronomy ! Definitely a career to be in .It took a week for the implications of the cross correlation function article to sink in for me and to link it with the closely related high dispersion spectroscopy technique that I’ve read a lot about without appreciating its significance or implication.

Better still, high dispersion spectroscopy can be used in conjunction with high contrast imaging to allow detailed characterisation of exoplanets from the ground to levels approaching what can be achieved from space.

I think we are now at a cross roads of discovery and it’s reassuring to know that despite severe budgetary constraints great things are soon to be achieved in exoplanet science not just in space but on the ground . All due to the development of brilliant hardware and then bringing it all together . The next decade is going to be roller coaster of a ride so hang on !

If you want to read more and with better explanation than my own crude effort , Ignas Snellen has published numerous excellent articles and PowerPoint / PDFs on high dispersion spectroscopy and it’s incredibly exciting combination with high contrast direct imaging from the ground .

Greetings to all of you.
I apologize if my question is not entirely relevant to the subject, the article.
I wonder (and ask you, the readers, and the creator of this “blog”) if you have made any scientific research on the possible presence of some satellite, circling the planet “F” to “55 Cancri”.
I know that this planet is in the “habitable zone” of this star, and being similar to Saturn, it is not very interesting, despite being in the habitable zone.
Thank you.
Greetings to all of you, from Antonio Tavani

Back again. Read the ENTIRE PAPER for about the FIFTH TIME! I THINK there is an ANSWER to the question I asked in my SECOND comment: The maximum barymetric pressure, if ( AND ONLY IF) the volcanic activity theory IS correct, is 100 bars (i.e. like VENUS), but is MOST LIKELY MUCH LOWER (i.e. like MARS). IN the INFRARED, the VARIABILITY of the radius is from 1.75 to 2.25Re, with an AVERAGE of 1.92Re. I STRONGLY RECOMMEND a vastly increased observation cycle in the OPTICAL (with MOST) and in the ULTRAVIOLET (with HUBBLE) to see whether 55 Cancri e can be classified as the first variable PLANET across a WIDE spectral range!

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last twelve years, this site coordinated its efforts with the Tau Zero Foundation. It now serves as an independent forum for deep space news and ideas. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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